Metallic complexes of various azo dyes may be divided into two classes: those in which the azo group doesn't participate in the complexation, and those in which the azo group is one of the coordination sites. These latter complexes have more commercial applications because they exhibit better light stability than similar complexes where the azo nitrogen is not involved in the complexation.
Metal complexes can usually have different ratios between ligands and metal ions, e.g. 1:1, 2:1, and 3:1, etc. Also, depending on the number of coordination sites, ligands can be monodentate, bidentate, tridentate etc. Usually, the more coordination sites in a ligand, the better the chemical stability of the complex. For a comprehensive review on metal complex colorants, including metallized azo dyes and pigments see “Metal Complexes as Specialty Dyes and Pigments” by P. Gregory in Comprehensive Coordination Chemistry II-From Biology to Nanotechnology Elsevier Ltd., 2003; Chapter 9.12, pp549–579.
If a ligand can form a 2 (or higher):1 (ligand:metal ratio) complex and if two or more ligand molecules are covalently bound such that an intramolecular complexation is not possible, upon metallation, this ligand could potentially form a multimetallic polymeric structure. These multimetallic polymeric azo colorants may have a wide range of molecular weights, potentially controlled by a variety of conditions: e.g. solubility in the polymerization/metallation reaction mixture, the amounts of monomeric ligands added. during metallation, rigidity of the covalent tether in the dimeric ligand, etc. Multimetallic polymeric azo dyes were reported previously by Banerjie et al (Vishwanath Banerjie, Arun K. Dey Makromol. Chem., Rapid Commun. 1, 41–46 (1980)) but the ligands described are bidentate (5,5′-(p-phenylenebisazo)diquinolin-8-ol) and the azo group is not a coordination site. The participation of the azo group as a coordination site is critical to the light stability and chemical (or thermal) stability of the dye.
There is still a need for a new class of multimetallic polymeric azo colorants which exhibit light and thermal stability. Such dyes have potential applications either as pigments or dyes in electrophotography, displays, plastics, coatings, recording media, nanotechnology, or inkjet inks.